Nonvolatile memory is a type of memory that retains stored data when power is removed from the memory. Thus, nonvolatile memory devices are being widely employed in computers, mobile communication terminals, memory cards, and the like. There are various types of nonvolatile memories including e.g., flash memory.
A flash memory device is commonly used as a nonvolatile memory device. In general, the flash memory device includes memory cells, each of which has a stacked gate structure. The stacked gate structure may include a tunnel oxide layer, a floating gate, an inter-gate dielectric layer, and a control gate electrode, which are sequentially stacked on a channel region.
Recently, new nonvolatile memory devices, such as a resistance random access memory (RRAM), have been proposed. A unit cell of the RRAM includes a data storage element which has two electrodes and a variable resistive material layer interposed between the two electrodes. The variable resistive material layer, i.e., a data storage material layer, has a reversible variation in resistance according to the whether a filament, a conductive path or a low resistive path is formed through the resistive material layer by the electrical signal (voltage or current) applied between the electrodes.
A new write mechanism, which is based upon spin polarization current induced magnetization switching, has been introduced to the RRAM design. This new RRAM design, called Spin-Transfer Torque RAM (STRAM), uses a (bidirectional) current through the MTJ to realize the resistance switching. Therefore, the switching mechanism of STRAM is constrained locally and STRAM is believed to have a better scaling property than the conventional RRAM.
While RRAM appears to be a promising nonvolatile memory, there are a number of challenges with RRAM. One illustrative challenge is that the distribution of the write pulse and of the cell resistance for an RRAM memory array appears to be wide, which results in difficulty in writing and reading the RRAM memory array.